The invention relates to the techniques for forming fibres from glass or other thermoplastics materials by an internal centrifugation process associated with a drawing by a high-temperature flow of gases. It is applied particularly to the industrial production of glass wool intended for use for example in the composition of heat and/or sound insulating products.
The fibre-forming process to which the invention relates consists in introducing a stream of molten glass into a centrifuge, also referred to as a fibre-forming platform, turning at high speed and provided on its periphery with a very large number of orifices through which the glass is sprayed in the form of filaments under the effect of centrifugal force. These filaments are then subjected to the action of an annular drawing current at elevated temperature and velocity flowing along the wall of the centrifuge which thins them out and converts them to fibres. Within the meaning of the invention, the term `elevated temperature and velocity` is understood to refer to a temperature at least in excess of 500.degree. C. and an annular flow velocity greater than or equal to 50 m/s. The fibres formed are entrained by this gaseous drawing current to a receiving device which quite generally consists of a strip which is permeable to gases. This method has been the subject of numerous improvements, including particularly those disclosed in the patents U.S. Pat. No. 2,991,507, FR-A-2 147 765, FR-A-2 459 783, FR-A-2 443 436, EP-A-91 381 and EP-A-91 866.
Although the velocity of the gaseous drawing current is very high and systematically greater than the velocity at which the filaments are projected, the kinetic energy in the fibres is sufficient that many of them penetrate the gaseous drawing current which surrounds the centrifuge to a thickness of just a few millimeters. This drawing current then spreads out just below the centrifuge which has the effect of dispersing the fibres over a wide surface area. Finally, these fibres change course to fall onto the receiving belt situated a few meters below. Thus, the receiving belt intercepts the fibres dispersed in a cylindrical torus having a diameter which is small in relation to the width of the belt, which makes it difficult to achieve a properly uniform distribution of the fibres over the belt.
Furthermore, the thermal balance in the centrifuge is more often than not provided by induction heating using an annular inductor through which an electric current passes. Maximum efficiency is achieved when this annular inductor is very close to the centrifuge. As the centrifuges used are preferably centrifuges with no bottom, this inductor can only be installed by disposing it just outside the centrifuge, in a concentric manner. Therefore, all that is left for passage of the fibres is a relatively narrow space but this must of necessity be retained in order to avoid the inductor constituting an obstacle which would obviously impair the quality of the end product and in any case it could not function properly for a long time because it would become blocked by the fibres striking and adhering to it.
To remedy this problem, it is known to confine the gaseous drawing current by means of a layer of cold gases enveloping and channelling it in a suitable fashion. This gaseous layer is produced by a blower ring surrounding the annular burner. The cold air makes it possible furthermore to assist the cooling of the fibres, the mechanical strength of which is thus improved by a heat-hardening effect.
This gaseous layer is generated for example by a blower ring similar to that described in U.S. Pat. No. 2,991,507, that is to say it is constituted by an annular tube provided with a circumferential slot or a series of orifices which are close to one another, the divergence of the jets ensuring continuity of the fluid barrier formed not later than at the height of the first row of orifices of the centrifuge, these rows being systematically counted from top to bottom by men skilled in the art. Thus, a sealing-tight barrier is formed which cannot be traversed by the fibres, which means that these latter are channelled.
Nevertheless, this confinement of the fibre layer does not resolve the problems of fibre distribution and in particular the problems due to the formation of tufts by tangling of the fibres. Before referring to the tuft formation again, it must be stressed that these lay behind many faults which are observed in the end products.
Firstly, these tufts constitute a locally heterogeneous fibre distribution and the greater the length of the tufts, the more noticeable the fault. The tuft has a tendency to become rolled onto itself and, by becoming so accumulated, to leave areas which are fibre deficient. In these areas, the product has a lesser mass per unit of surface area which locally changes the properties of the product. To guarantee a minimum performance level, therefore, it is necessary to compensate for these locally fibre deficient areas by providing an excess of fibres which renders the cost of the product much greater.
Furthermore, the orientation of the fibres in the tufts differs from the general orientation of isolated fibres which may itself differ from the orientation which is desired in the end product. Therefore, the tufts will complicate the control of this final orientation which will in particular affect the insulating properties, the propensity for delamination and the resistance to crushing.
Furthermore, these tufts often form very high up in the fibre-forming and receiving hood before the fibres have been sprayed with binder. If the fibres are not properly isolated when the glue is applied, the distribution of the binder is not completely homogeneous and the fibres which are not thus sized are inclined to show up in the end product in the form of white spots which contrast with the fibres coloured by the binder. The appearance of the product is not greatly affected but above all certain mechanical properties such as for example the tensile strength, the resistance to fibre tearing, the rigidity, the resumption of former size and the aptitude for cutting will all be affected.
All these parameters play a more or less important part according to the type of products which are generally classified as light products--in which the density is less than 25 or even 15 kg/m.sup.3 --generally available in the form of rolls, or heavy products--the density of which is typically greater than 30 kg/m.sup.3 and which are often subject to conditions of use which entail good mechanical strength. It must furthermore be stressed that although the properties desired for heavy or light products may differ only slightly, it is desirable to have polyvalent production lines, in other words lines having means which tend to resolve the problems posed at the most upstream point possible and not purely palliatives which remedy only a few defects which are specific to heavy or alternatively light products.
Thinking along these lines, therefore, the solution is not to be found solely in mechanical or pneumatic distributor means such as those described in the patents EP-A 69 321 and EP-A 125 963 and which set out to cause a movement aimed at balancing the fibre torus. Indeed, such means are only effective from the point of view of the final distribution of the surface masses but not in terms of the actual tufting and they are quite particularly inappropriate with regard to long tuft problems.
It should be noted furthermore that such means often require prolonged and delicate adjustment which can only be carried out by skilled personnel and which must furthermore be repeated whenever there is a change in production. Furthermore, it must be stressed that the difficulty of such adjustment tends to make it virtually impossible to isolate the factors involved in the mechanisms as a whole, in the blower ring which in particular plays a certain part in the process of forming fibres and tufts, inseparably supplementing its role of confinement of the fibre layer.